Interval
Estimation for the Difference
Between Two Proportions
This
approach uses sample data to determine a range (interval) that, at an established level of
confidence, will contain the difference between two population proportions.
Steps
Determine
the confidence level (generally alpha
.05)
Use
the z distribution table to find the critical value for a 2-tailed test (at alpha
.05 the critical value would equal 1.96)
Estimate
Sampling Error
where
 and
Estimate
the Interval
CI
= (p1-p2) ± (CV)(Sp1-p2)
p1-p2
= difference between two sample proportions
CV
= critical value
Interpret
Based
on alpha
.05, you are 95% confident that the difference between the proportions of the two
subgroups in the population from which the sample was obtained is between __ and __.
Note:
Given the sample data and level of error, the confidence interval provides an estimated
range of proportions that is most likely to contain the difference between the population
subgroups. The term "most likely" is measured by alpha
or in most cases there is a 5% chance (alpha .05) that the confidence interval does not
contain the true difference between the subgroups in the population.
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